Fossil Record Shows Species Need to Spread Out to Survive

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Counter to expectations, groups of related marine species with
large population sizes have just the same risk of extinction as
those with small population sizes, according to new research.

The new analysis of marine fossil records across 500 million
years shows that ocean invertebrates (animals without backbones)
limited to small geographic ranges typically bear the brunt of
extinction, regardless of their abundance there. The findings
offer a potentially important clue for present-day
conservationists.

"There's this generally widely held belief that species that are
more rare should be more vulnerable to extinction," lead study
author Paul Harnik, a postdoctoral fellow at the National
Evolutionary Synthesis Center in Durham, N.C., told LiveScience.
"The average population size has no association with
extinction risk over the history of marine animals."

Risk of extinction

Harnik and researchers at Stanford University and Humboldt
University used the Paleobiology Database, a community-based
fossil database, to analyze the global marine fossil record.

The researchers focused on 6,500 groups of related species,
called genera, of
sea urchins, sand dollars, corals, snails, clams, oysters,
scallops, brachiopods and other invertebrates. They examined
which organisms disappeared from the fossil record, along with
their geographic range, variety of habitats and relative
population numbers. [ Image
Gallery: Quirky Sea Creatures ]

In all, they examined records of hundreds of thousands of fossil
observations from over 42,000 collections at museums and
universities.

"We see a huge variation in population size in the fossil record,
and yet it's not associated with extinction," Harnik said.

Compared with genera that lived both far and wide, related groups
of species that could occupy only small regions of the globe had
a six-fold chance of becoming extinct.

Of the genera limited to small regions, however, the ability to
live in a variety in habitats decreased the risk of extinction by
30 percent.

Implications for conservation

"The findings don't mean that when populations dwindle we
shouldn't worry about them," Harnik said. Instead, limiting a
species' range through
habitat destruction or degradation could raise the risk of
extinction, even for a species with a relatively large
population.

However, what once drove extinction may no longer be relevant in
an era in which vanishing species are more common, one
conservation expert noted.

"Finding small geographic range size to be a driver of risk is
par for the course ... in the fossil record. But if dinosaurs
drove fishing trawlers, the pattern would be different and more
like what we see today, with the largest-bodied species suffering
most," Nicholas Dulvy, marine biodiversity and conservation
professor at Simon Fraser University, wrote in an email.

"While historic drivers of fossil extinctions have been vast —
meteorites and volcanism — they have never been as unprecedented
as the unique impact of the growing humanity driving the
sixth mass extinction — the Anthropocene," added Dulvy, who
was not involved in the current study.

Today's extinction rates, he noted, are one to two times the
magnitude of those in the fossil record, with future rates
expected to climb further.

"There is a real risk that the threatening processes that have
gone before may tell us little about what is to come," Dulvy
said.

The study is detailed today (Oct. 23) in the journal Proceedings
of the Royal Society B.